Androgen exposure impairs neutrophil maturation and function within the infected kidney

Urinary tract infections (UTIs) in men are uncommon yet carry an increased risk for severe pyelonephritis and other complications. In models of Escherichia coli UTI, C3H/HeN mice develop high-titer pyelonephritis (most with renal abscesses) in a testosterone-dependent manner, but the mechanisms underlying this phenotype are unknown. Here, using female mouse models, we show that androgen exposure impairs neutrophil maturation in the upper and lower urinary tract, compounded by a reduction of neutrophil function within the infected kidney, enabling persistent high-titer infection and promoting abscess formation. Following intravesical inoculation with uropathogenic E. coli (UPEC), kidneys of androgen-exposed C3H mice showed delayed local pro-inflammatory cytokine responses while robustly recruiting neutrophils. These were enriched for an end-organ-specific population of aged but immature neutrophils (CD49d+, CD101-). Compared to their mature counterparts, these aged immature kidney neutrophils exhibited reduced function in vitro, including impaired degranulation and diminished phagocytic activity, while splenic, bone marrow, and bladder neutrophils did not display these alterations. Furthermore, aged immature neutrophils manifested little phagocytic activity within intratubular UPEC communities in vivo. Experiments with B6 conditional androgen receptor (AR)-deficient mice indicated rescue of the maturation defect when AR was deleted in myeloid cells. We conclude that the recognized enhancement of UTI severity by androgens is attributable, at least in part, to local impairment of neutrophil maturation in the urinary tract (largely via cell-intrinsic AR signaling) and a kidney-specific reduction in neutrophil antimicrobial capacity.IMPORTANCEAlthough urinary tract infections (UTIs) predominantly occur in women, male UTIs carry an increased risk of morbidity and mortality. Pyelonephritis in androgen-exposed mice features robust neutrophil recruitment and abscess formation, while bacterial load remains consistently high. Here, we demonstrate that during UTI, neutrophils infiltrating the urinary tract of androgen-exposed mice exhibit reduced maturation, and those that have infiltrated the kidney have reduced phagocytic and degranulation functions, limiting their ability to effectively control infection. This work helps to elucidate mechanisms by which androgens enhance UTI susceptibility and severity, illuminating why male patients may be predisposed to severe outcomes of pyelonephritis.

Androgen exposure alters the neutrophil response to pyelonephritis

Urinary tract infections (UTIs) in men are uncommon but carry increased risk for severe upper-tract UTI (pyelonephritis) and complications including renal scarring. In preclinical models of Escherichia coli UTI, male C3H/HeN mice uniformly develop high-titer pyelonephritis (most with renal abscesses), while the majority of female C3H/HeN mice resolve renal infection within 7–14 days. We previously showed that this difference in outcomes is testosterone dependent. Here, we find that androgen exposure in female C3H/HeN mice alters the neutrophil response to UTI, increasing the propensity for severe renal inflammation and abscess formation as opposed to bacterial clearance and resolution of infection. Concordant with our prior published results, androgen exposure in C3H female mice given Escherichia coli UTI led to 3-log higher bacterial loads in the kidneys by 14 days post infection. We found that the kidneys of androgen-exposed mice harbored more neutrophils than standard females at baseline and throughout infection, correlated with higher levels of the neutrophil-recruiting chemokine CXCL1 (murine KC) in kidney tissue. In addition, the kidneys of androgen-exposed females were enriched for aged, immature neutrophils (Ly6G+, CD49d+, CD101−). Compared to their mature (CD101+) counterparts, these cells exhibited markers of increased degranulation, altered phagocytic activity, and diminished capacity for efflux from kidney tissue. These data support a model in which androgen-exposed mice, despite an apparently more robust neutrophil presence, fail to control renal bacterial infection due to altered neutrophil functions.

Supported by grants from NIH (R01 DK111541, R01 DK126697, R01 AI158418)

Morphologic Design of Silver-Bearing Sugar-Based Polymer Nanoparticles for Uroepithelial Cell Binding and Antimicrobial Delivery

Platelet-like and cylindrical nanostructures from sugar-based polymers are designed to mimic the aspect ratio of bacteria and achieve uroepithelial cell binding and internalization, thereby improving their potential for local treatment of recurrent urinary tract infections. Polymer nanostructures, derived from amphiphilic block polymers composed of zwitterionic poly(d-glucose carbonate) and semicrystalline poly(l-lactide) segments, were constructed with morphologies that could be tuned to enhance uroepithelial cell binding. These nanoparticles exhibited negligible cytotoxicity, immunotoxicity, and cytokine adsorption, while also offering substantial silver cation loading capacity, extended release, and in vitro antimicrobial activity (as effective as free silver cations) against uropathogenic Escherichia coli. In comparison to spherical analogues, cylindrical and platelet-like nanostructures engaged in significantly higher association with uroepithelial cells, as measured by flow cytometry; despite their larger size, platelet-like nanostructures maintained the capacity for cell internalization. This work establishes initial evidence of degradable platelet-shaped nanostructures as versatile therapeutic carriers for treatment of epithelial infections.

Androgen-Influenced Polarization of Activin A-Producing Macrophages Accompanies Post-pyelonephritic Renal Scarring

Ascending bacterial pyelonephritis, a form of urinary tract infection (UTI) that can result in hospitalization, sepsis, and other complications, occurs in ~250,000 US patients annually; uropathogenic Escherichia coli (UPEC) cause a large majority of these infections. Although UTIs are primarily a disease of women, acute pyelonephritis in males is associated with increased mortality and morbidity, including renal scarring, and end-stage renal disease. Preclinical models of UTI have only recently allowed investigation of sex and sex-hormone effects on pathogenesis. We previously demonstrated that renal scarring after experimental UPEC pyelonephritis is augmented by androgen exposure; testosterone exposure increases both the severity of pyelonephritis and the degree of renal scarring in both male and female mice. Activin A is an important driver of scarring in non-infectious renal injury, as well as a mediator of macrophage polarization. In this work, we investigated how androgen exposure influences immune cell recruitment to the UPEC-infected kidney and how cell-specific activin A production affects post-pyelonephritic scar formation. Compared with vehicle-treated females, androgenized mice exhibited reduced bacterial clearance from the kidney, despite robust myeloid cell recruitment that continued to increase as infection progressed. Infected kidneys from androgenized mice harbored more alternatively activated (M2) macrophages than vehicle-treated mice, reflecting an earlier shift from a pro-inflammatory (M1) phenotype. Androgen exposure also led to a sharp increase in activin A-producing myeloid cells in the infected kidney, as well as decreased levels of follistatin (which normally antagonizes activin action). As a result, infection in androgenized mice featured prolonged polarization of macrophages toward a pro-fibrotic M2a phenotype, accompanied by an increase in M2a-associated cytokines. These data indicate that androgen enhancement of UTI severity and resulting scar formation is related to augmented local activin A production and corresponding promotion of M2a macrophage polarization.

TGFβ1 orchestrates renal fibrosis following Escherichia coli pyelonephritis

Renal scarring after pyelonephritis is linked to long-term health risks for hypertension and chronic kidney disease. Androgen exposure increases susceptibility to, and severity of, uropathogenic Escherichia coli (UPEC) pyelonephritis and resultant scarring in both male and female mice, while anti-androgen therapy is protective against severe urinary tract infection (UTI) in these models. This work employed androgenized female C57BL/6 mice to elucidate the molecular mechanisms of post-infectious renal fibrosis and to determine how these pathways are altered by the presence of androgens. We found that elevated circulating testosterone levels primed the kidney for fibrosis by increasing local production of TGFβ1 before the initiation of UTI, altering the ratio of transcription factors Smad2 and Smad3 and increasing the presence of mesenchymal stem cell (MSC)-like cells and Gli1 + activated myofibroblasts, the cells primarily responsible for deposition of scar components. Increased production of TGFβ1 and aberrations in Smad2:Smad3 were maintained throughout the course of infection in the presence of androgen, correlating with renal scarring that was not observed in non-androgenized female mice. Pharmacologic inhibition of TGFβ1 signaling blunted myofibroblast activation. We conclude that renal fibrosis after pyelonephritis is exacerbated by the presence of androgens and involves activation of the TGFβ1 signaling cascade, leading to increases in cortical populations of MSC-like cells and the Gli1 + activated myofibroblasts that are responsible for scarring.

Mitochondrial inefficiency in infants born to overweight African-American mothers

Background

Currently 20–35% of pregnant women are obese, posing a major health risk for mother and fetus. It is postulated that an abnormal maternal-fetal nutritional environment leads to adverse metabolic programming, resulting in altered substrate metabolism in the offspring and predisposing to risks of obesity and diabetes later in life. Data indicate that oocytes from overweight animals have abnormal mitochondria. We hypothesized that maternal obesity is associated with altered mitochondrial function in healthy neonatal offspring.

Methods

Overweight and obese (Body mass index, (BMI) ≥ 25 kg/m², n=14) and lean (BMI < 25 kg/m², n=8), African American pregnant women carrying male fetuses were recruited from the Barnes Jewish Hospital obstetric clinic. Maternal and infant data were extracted from medical records. Infants underwent body composition testing in the first days of life. Circumcision skin was collected for isolation of fibroblasts. Fibroblast cells were evaluated for mitochondrial function, metabolic gene expression, nutrient uptake and oxidative stress.

Results

Skin fibroblasts of infants born to overweight mothers had significantly higher mitochondrial respiration without a concurrent increase in ATP production, indicating mitochondrial inefficiency. These fibroblasts had higher levels of reactive oxygen species and evidence of oxidative stress. Evaluation of gene expression in offspring fibroblasts revealed altered expression of multiple genes involved in fatty acid and glucose metabolism and mitochondrial respiration in infants of overweight mothers.

Conclusion

This study demonstrates altered mitochondrial function and oxidative stress in skin fibroblasts of infants born to overweight mothers. Future studies are needed to determine the long-term impact of this finding on the metabolic health of these children.

High-sucrose-induced maternal obesity disrupts ovarian function and decreases fertility in Drosophila melanogaster

As the obesity epidemic worsens, the prevalence of maternal obesity is expected to rise. Both high-fat and high-sucrose diets are known to promote maternal obesity and several studies have elucidated the molecular influence of high-fat feeding on female reproduction. However, to date, the molecular impact of a high-sucrose diet on maternal obesity remains to be investigated. Using our previously reported Drosophila high-sucrose maternal obesity model, we sought to determine how excess dietary sucrose impacted the ovary. High-sucrose diet (HSD) fed adult females developed systemic insulin resistance and exhibited an ovarian phenotype characterized by excess accumulation of lipids and cholesterol in the ovary, decreased ovary size, and impaired egg maturation. We also observed decreased expression of antioxidant genes and increased protein carbonylation in the ovaries of HSD females. HSD females laid fewer eggs; however, the overall survival of offspring was unchanged relative to lean control females. Ovaries of HSD females had increased mitochondrial DNA copy number and decreased expression of key mitochondrial regulators, suggestive of an ineffective compensatory response to mitochondrial dysfunction. Mitochondrial alterations were also observed in male offspring of obese females. This study demonstrates that high-sucrose-induced maternal obesity promotes insulin resistance, while disrupting ovarian metabolism and function.

Role of beta-catenin in epidermal stem cell expansion, lineage selection, and cancer

The mammalian epidermis is an excellent model with which to analyze the factors that regulate adult stem cell renewal, lineage selection, and tumor formation. One of the key regulators of all three processes is beta-catenin, the main cytoplasmic effector of the canonical Wnt signaling pathway. In this chapter, we review some of the ways in which beta-catenin exerts its effects on cultured human epidermal cells and in genetically modified mice. We highlight the importance of the timing and level of activation and discuss some of the pathways activated downstream from beta-catenin. Finally, we demonstrate the importance of Lef/Tcf-independent beta-catenin signaling through interaction with the vitamin D receptor.

Processing, characterisation and biocompatibility of iron-phosphate glass fibres for tissue engineering

Iron-phosphate glass fibres based on the CaO-Na2O-Fe2O3-P2O5 system have been processed and characterised via thermal, XRPD, dissolution rates, diameter and biocompatibility studies. The compositions investigated were fixed at 50mol% P2O5, and the CaO content was varied between 30, 35 and 40mol%. The Fe2O3 was added in low amounts from 1-5mol%, substituting it for the Na2O mol%. The number of Tc (crystallisation temperature) peaks detected from the thermal analysis traces only showed correlation with XRPD analysis, for five out of the 15 compositions investigated. It has been suggested that either the crystalline phases had very similar Tc temperatures or that the other phase(s) were present in very small quantities. There was a good match seen with number of Tm (melting temperature) peaks picked up from the DTA traces, with the number of phases identified from XRPD analysis. The main phases identified from XRPD were NaCa(PO3)3, CaP2O6 and NaFeP2O7. Using network connectivity (NC), predictions on Qn species present within the compositions investigated were made. The predicted species (metaphosphates) matched with phases identified from XRPD analysis. A decrease in dissolution rates for the bulk glass and glass fibres was seen with an increase in CaO mol%, along with an increase in Fe2O3 mol%. An increase in fibre dissolution rates was seen with a decrease in diameter size. The biocompatibility studies were conducted using a conditionally immortal muscle precursor cell line derived from the H-2Kb-tsA58 immortomouse. It was found that iron-phosphate glass fibres containing 4-5mol% Fe2O3 was sufficient for cell attachment and differentiation. It was seen that myotubes formed along the axis of the fibres (which was indicative of differentiation). The biocompatibility of these compositions was attributed to the enhanced chemical durability of the glass fibres.

Duchenne's muscular dystrophy: animal models used to investigate pathogenesis and develop therapeutic strategies

Duchenne's muscular dystrophy (DMD) is a lethal childhood disease caused by mutations of the dystrophin gene, the protein product of which, dystrophin, has a vital role in maintaining muscle structure and function. Homologues of DMD have been identified in several animals including dogs, cats, mice, fish and invertebrates. The most notable of these are the extensively studied mdx mouse, a genetic and biochemical model of the human disease, and the muscular dystrophic Golden Retriever dog, which is the nearest pathological counterpart of DMD. These models have been used to explore potential therapeutic approaches along a number of avenues including gene replacement and cell transplantation strategies. High-throughput screening of pharmacological and genetic therapies could potentially be carried out in recently available smaller models such as zebrafish and Caenorhabditis elegans. It is possible that a successful treatment will eventually be identified through the integration of studies in multiple species differentially suited to addressing particular questions.

Genetic interactions between the 5' and 3' splice site consensus sequences and U6 snRNA during the second catalytic step of pre-mRNA splicing

The YAG/ consensus sequence at the 3' end of introns (the slash indicates the location of the 3' splice site) is essential for catalysis of the second step of pre-mRNA splicing. Little is known about the interactions formed by these three nucleotides in the spliceosome. Although previous observations have suggested that the G of the YAG/ interacts with the first nucleotide of the /GUA consensus sequence at the 5' end of the intron, additional interactions have not been identified. Here we report several striking genetic interactions between A+3 of the 5' /GUA with Y-3 of the 3' YAG/ and G50 of the highly conserved ACAGAG motif in U6 snRNA. Two mutations in U6 G50 of the ACAGAG can weakly suppress two mutations in A+3 of the 5' /GUA. This suppression is significantly enhanced upon the inclusion of a specific mutation Y-3 in the 3' YAG/. RNA analysis confirmed that the severe splicing defect observed in A+3 and Y-3 double mutants can be rescued to near wild-type levels by the mutations in U6 G50. The contributions of each mutation to the genetic interaction and the strong position specificity of suppression, combined with previous findings, support a model in which the 5' /GUA and the GAG of U6 function in binding the 3' YAG/ during the second catalytic step.

Form or function? Part 2. Objective cosmetic and functional correlates of quality of life in women treated with breast-conserving surgical procedures and radiotherapy

BACKGROUND

Treatment-related factors that influence quality of life (QOL) for women who are diagnosed with breast carcinoma require study. This study was designed to examine the convergent validity of objective and subjective indices of cosmetic and functional status after patients undergo breast-conserving treatment (BCT) and to test the relations of the objective indicators with QOL.

METHODS

Women (n = 54 patients) who had received BCT and radiotherapy for Stage 0-II disease for whom the diagnosis duration ranged from 9 months to 18 years completed assessments of background information, perceived cosmetic and functional outcomes, and QOL. They also were administered a clinical breast examination, including objective ratings of arm edema and breast cosmesis. The patients were a subsample from the study reported by the authors in an accompanying article that is presented in this issue.

RESULTS

The findings revealed positive cosmetic and functional treatment outcomes, such that 82% of patients had no or minimal arm edema, and 70% of patients evidenced good or excellent cosmetic results. Convergent validity was demonstrated for the objective and subjective assessments of cosmetic and functional treatment outcomes. In addition, women who had more arm edema reported poorer QOL with regard to depressive symptoms (P < 0.05) and fear of disease recurrence (P < 0.05).

CONCLUSIONS

The findings from this study and those reported in the accompanying article reveal that functional treatment outcomes, such as arm edema and breast specific pain, are important correlates of QOL even many years after patients undergo BCT and radiotherapy. Both subjective and objective indicators of treatment outcomes are useful predictors of QOL.

Form or function? Part 1. Subjective cosmetic and functional correlates of quality of life in women treated with breast-conserving surgical procedures and radiotherapy

BACKGROUND

Quality of life (QOL) after a diagnosis of cancer varies considerably across individuals. Treatment-related factors that predict QOL for women who are diagnosed with breast carcinoma require further specification. This study was designed to develop a measure of perceived aesthetic (e.g., breast shape) and functional status (e.g., pain, mobility) after breast-conserving surgical treatment (BCT) and radiotherapy, to examine the relations of these indicators with patient QOL, and to determine whether these relations varied as a function of diagnosis duration.

METHODS

Women (n = 185 patients) who underwent BCT and radiotherapy for Stage 0-II disease for whom the diagnosis duration ranged from 3 months to 18 years completed assessments of background information, perceived cosmetic and functional treatment outcomes, and QOL. Medical data also were obtained from medical charts.

RESULTS

The Breast Cancer Treatment Outcome Scale (BCTOS) produced a coherent factor structure and three internally consistent subscales (i.e., cosmetic status, functional status, and breast specific pain) that demonstrated predictive validity. With patient age, diagnosis duration, and other BCTOS subscales controlled, greater breast specific pain predicted greater depressive symptoms (P < 0.01) and lower QOL related to mental health (P < 0.05) and physical health (P < 0.05). Cosmetic status predicted QOL related to physical health (P < 0.05). The relations of breast specific pain with QOL indicators varied somewhat as a function of diagnosis duration.

CONCLUSIONS

Although considerable research on treatment-relevant outcomes has addressed appearance-related concerns, functional parameters have not been explored fully. Findings suggest that functional consequences of treatment, and particularly breast specific pain, also are significant influences on patient QOL.

Modulation of cytoplasmic dynein ATPase activity by the accessory subunits

The microtubule-based motor molecule cytoplasmic dynein has been proposed to be regulated by a variety of mechanisms, including phosphorylation and specific interaction with the organelle-associated complex, dynactin. In this study, we examined whether the intermediate chain subunits of cytoplasmic dynein are involved in modulation of ATP hydrolysis, and thereby affect motility. Treatment of testis cytoplasmic dynein under hypertonic salt conditions resulted in separation of the intermediate chains from the remainder of the dynein molecule, and led to a 4-fold enhancement of ATP hydrolysis. This result suggests that the accessory subunits act as negative regulators of dynein heavy chain activity. Comparison of ATPase activities of dyneins with differing intermediate chain isoforms showed significant differences in basal ATP hydrolysis rates, with testis dynein 7-fold more active than dynein from brain. Removal of the intermediate chain subunits led to an equalization of ATPase activity between brain and testis dyneins, suggesting that the accessory subunits are responsible for the observed differences in tissue activity. Finally, our preparative procedures have allowed for the identification and purification of a 1:1 complex of dynein with dynactin. As this interaction is presumed to be mediated by the dynein intermediate chain subunits, we now have defined experimental conditions for further exploration of dynein enzymatic and motility regulation.

Emotionally expressive coping predicts psychological and physical adjustment to breast cancer

This study tested the hypothesis that coping through emotional approach, which involves actively processing and expressing emotions, enhances adjustment and health status for breast cancer patients. Patients (n = 92) completed measures within 20 weeks following medical treatment and 3 months later. Women who, at study entry, coped through expressing emotions surrounding cancer had fewer medical appointments for cancer-related morbidities, enhanced physical health and vigor, and decreased distress during the next 3 months compared with those low in emotional expression, with age, other coping strategy scores, and initial levels on dependent variables (except medical visits) controlled statistically. Expressive coping also was related to improved quality of life for those who perceived their social contexts as highly receptive. Coping through emotional processing was related to one index of greater distress over time. Analyses including dispositional hope suggested that expressive coping may serve as a successful vehicle for goal pursuit.

The question remains: is the spliceosome a ribozyme?

The two phosphoryl transfer steps of pre-mRNA splicing are catalyzed within the large ribonuclear protein machine called the spliceosome. The highly dynamic nature of the spliceosome has presented many challenges to a structural and mechanistic understanding of its catalytic core. While much evidence supports the popular hypothesis that the catalytic steps of pre-mRNA splicing are mediated by spliceosomal RNA, a role for protein in catalysis cannot yet be ruled out. A highly conserved protein, Prp8, is a component of the catalytic core. We review data consistent with the hypothesis that Prp8 functions as a cofactor to an RNA enzyme.

Cluster Selection and the Evolution of Brightest Cluster Galaxies

The K-band Hubble diagram of brightest cluster galaxies (BCGs) is presented for a large, X-ray-selected cluster sample extending out to z=0.8. The controversy over the degree of BCG evolution is shown to be due to sample selection since the BCG luminosity depends on the cluster environment. Selecting only the most X-ray luminous clusters produces a BCG sample that shows, under the assumption of an Einstein-de Sitter cosmology, significantly less mass growth than that predicted by current semianalytic galaxy formation models, and significant evidence of any growth only if the dominant stellar population of the BCGs formed relatively recently (z</=2.6).

Allele-specific genetic interactions between Prp8 and RNA active site residues suggest a function for Prp8 at the catalytic core of the spliceosome

The highly conserved spliceosomal protein Prp8 is known to cross-link the critical sequences at both the 5' (GU) and 3' (YAG) ends of the intron. We have identified prp8 mutants with the remarkable property of suppressing exon ligation defects due to mutations in position 2 of the 5' GU, and all positions of the 3' YAG. The prp8 mutants also suppress mutations in position A51 of the critical ACAGAG motif in U6 snRNA, which has been observed previously to cross-link position 2 of the 5' GU. Other mutations in the 5' splice site, branchpoint, and neighboring residues of the U6 ACAGAG motif are not suppressed. Notably, the suppressed residues are specifically conserved from yeast to man, and from U2- to U12-dependent spliceosomes. We propose that Prp8 participates in a previously unrecognized tertiary interaction between U6 snRNA and both the 5' and 3' ends of the intron. This model suggests a mechanism for positioning the 3' splice site for catalysis, and assigns a fundamental role for Prp8 in pre-mRNA splicing.

Suspected spinal cord compression in breast cancer patients: a multidisciplinary risk assessment

Breast cancer is the most common cause of metastatic epidural spinal cord compression (SCC) in women, and this condition results in significant neurologic dysfunction and morbidity. Prior studies of patients with suspected SCC did not employ multivariate analysis techniques, often included persons with a wide variety of malignancies, and generally focused on identifying associated neurologic and radiologic features. We therefore conducted a study examining a more comprehensive set of potential clinical risk factors in breast cancer patients with suspected SCC. We retrospectively analysed 123 episodes of suspected SCC among 93 breast cancer patients evaluated by spine computed tomography (CT) scanning. Multiple logistic regression analysis was employed to identify independent predictors of SCC. Clinically significant metastatic epidural cancer was defined as thecal sac compression (TSC), which occurred in 33 episodes (27%). Four independent predictors of TSC were identified and included oncologic features (known bone metastases > or = 2 years, metastatic disease at initial diagnosis) in addition to neurologic and radiologic features (objective weakness, vertebral compression fracture on spine radiograph). These four predictors stratified episodes into subgroups with widely varying risks of TSC, ranging from 12% (0 risk factors) to 85% (> or = 3 risk factors). These results suggest that the evaluation of breast cancer patients with suspected SCC should include clinical information about their disease course in addition to neurologic examination and prior imaging studies. If confirmed, these predictors may help clinicians assess risk in this patient population.

Comparison of the intracellular distribution of cytoplasmic dynein and kinesin in cultured cells: motor protein location does not reliably predict function

While immunolocalization methods have been used as a reasonable means to judge where a given molecule may be active in the cellular milieu, the correlation between distribution and function for proteins involved in intracellular transport may not be clear cut. To address the question of specificity and reproducibility of immunolocalization of microtubule-based motor proteins, we have co-localized cytoplasmic dynein and kinesin by immunofluorescence microscopy using two specific antibodies for each motor molecule. The results indicate that cytoplasmic dynein and kinesin appear to co-localize on a small subset of vesicles, but largely reside or accumulate on morphologically distinct organelles. In addition, anti-kinesin antibodies differing in their epitope specificity label different cellular compartments. To address the question of whether the distribution of motor molecules is representative of organelles that are undergoing active transport, we have altered the activity of vesicle trafficking pathways in fibroblasts using several different methods, including cytoplasmic acidification and disruption of cellular compartments with brefeldin A, nocodazole and okadaic acid. Analysis of the distribution of cytoplasmic dynein and kinesin under these conditions indicates that immunolocalization data alone are not reliable indicators of sites of likely function for these microtubule-based motors.

Dynamin forms polymeric complexes in the presence of lipid vesicles. Characterization of chemically cross-linked dynamin molecules

Dynamin is a GTP-binding protein that is involved in the release of coated endocytic vesicles from the plasma membrane. We have been characterizing the enzymatic properties of purified rat brain dynamin to better understand how GTP binding and hydrolysis relate to its proposed function. Previously, we have demonstrated that activation of dynamin GTPase results from positive cooperative associations between dynamin molecules as they are bound to a polymeric surface. Our present report has extended these studies and has examined the structural features of dynamin self-association. After treatment with the zero-length protein cross-linking reagent, 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide, dynamin in solution was found cross-linked into dimers. This homodimer likely reflects the native soluble state of the molecule. After binding to brain vesicles, dynamin was cross-linked into higher order oligomers of greater than 800 kDa. Dynamin, copurified on brain membranous organelles, also formed multimeric complexes when cross-linked suggesting dynamin exists in polymeric form in vivo. No cross-linked species other than homo-oligomers were observed, providing no evidence for close interactions between dynamin and membrane proteins. From experiments examining the effects of GTP, GDP, guanosine 5'-3-O-(thio)triphosphate, and 5'-guanylyl-beta,gamma-imidodiphosphate on cross-linking, we have determined that both dynamin membrane binding and self-association occur independently from the nucleotide-bound state of the enzyme. An 80-kDa dynamin fragment that is lacking its carboxyl-terminal domain is not cross-linked into higher order oligomers, suggesting that this domain is required for binding of dynamin to membranes and the subsequent enhancement of oligomerization. However, the dynamin fragment was found to form dimers indicating that this domain is not required for dynamin dimerization. Cross-linked dynamin was able to cooperatively bind microtubules, but did not exhibit GTPase activation. We propose that intramolecular cross-links in the dynamin monomer impart structural constraints that prevent the enhancement of GTP hydrolysis. We describe a model of the dynamin activation process to be considered in further investigations of the role for dynamin in endocytic vesicle formation.

Microtubule-independent phospholipid stimulation of cytoplasmic dynein ATPase activity

In this study we report that phospholipid vesicles activate ATP hydrolysis by cytoplasmic dynein but not kinesin, consistent with reported differences in the organelle/vesicle binding of these motor proteins. Dynein activation by phospholipids was comparable with that seen in the presence of microtubules but was not sensitive to moderate salt concentrations and was independent of the net charge of the phospholipid, suggesting that the means of interaction between dynein and the lipid vesicle was not strictly ionic in nature. Based on this result, previous data that show that the interaction between dynein and vesicles is not ATP sensitive, and the concentration dependence observed for lipid activation of cytoplasmic dynein, it is likely that the binding interaction between dynein and liposomes is a stable one. In contrast to a previous report, microtubules increased the hydrolysis rate of all naturally occurring nucleotides tested, whereas only ATPase activity was stimulated by phospholipids. As ATP is the physiologically relevant substrate and is the only nucleotide to promote motility, the activation of only the ATPase by phospholipids may represent a means of discriminating between coupled and uncoupled nucleotide hydrolysis in vitro.

Activation of dynamin GTPase is a result of positive cooperativity

Dynamin is a GTP-binding protein thought to be involved in the early stages of endocytosis. Presently, it is not known how dynamin GTP binding and hydrolysis are related to its role in this process. We previously characterized the ability of acidic phospholipid vesicles and microtubules to strongly stimulate the GTPase activity of purified brain dynamin. In a further analysis of dynamin enzymatic properties, we have found that the increase of dynamin GTP hydrolysis in the presence of activating agent depends on enzyme concentration. At low enzyme concentration, little or no activation is observed. Plots of dynamin GTPase activity with increasing enzyme concentration in the presence of either activating agent are strongly sigmoidal, indicating that positive cooperativity is responsible for the increased activity observed. A Hill coefficient of 2.3 was determined, implying that at least two dynamin molecules associate for maximal GTPase activity. No cooperative effects in GTP binding were observed. Linear transformation of reaction velocity versus enzyme concentration data indicate an apparent Km for dynamin-dynamin interactions of 37 nM, which is significantly lower than the physiological concentration of dynamin in brain. These results suggest that cooperative interactions between dynamin molecules are responsible for the apparent activation of GTPase observed and are likely involved in dynamin function in vivo.

Cytoplasmic dynein undergoes intracellular redistribution concomitant with phosphorylation of the heavy chain in response to serum starvation and okadaic acid

Cytoplasmic dynein is a microtubule-binding protein which is considered to serve as a motor for retrograde organelle movement. In cultured fibroblasts, cytoplasmic dynein localizes primarily to lysosomes, membranous organelles whose movement and distribution in the cytoplasm have been shown to be dependent on the integrity of the microtubule cytoskeleton. We have recently identified conditions which lead to an apparent dissociation of dynein from lysosomes in vivo, indicating that alterations in membrane binding may be involved in the regulation of retrograde organelle movement (Lin, S. X. H., and C. A. Collins. 1993. J. Cell Sci. 105:579-588). Both brief serum withdrawal and low extracellular calcium levels induced this alteration, and the effect was reversed upon addition of serum or additional calcium. Here we demonstrate that the phosphorylation state of the dynein molecule is correlated with changes in its intracellular distribution in normal rat kidney fibroblasts. Dynein heavy chain phosphorylation level increased during serum starvation, and decreased back to control levels upon subsequent addition of serum. We found that okadaic acid, a phosphoprotein phosphatase inhibitor, mimicked the effects of serum starvation on both phosphorylation and the intracellular redistribution of dynein from a membrane-associated pool to one that was more soluble, with similar dose dependence for both phenomena. Cell fractionation by differential detergent extraction revealed that a higher proportion of dynein was present in a soluble pool after serum starvation than was found in comparable fractions from control cells. Our data indicate that cytoplasmic dynein is phosphorylated in vivo, and changes in phosphorylation state may be involved in a regulatory mechanism affecting the distribution of this protein among intracellular compartments.

Activation of dynamin GTPase by acidic phospholipids and endogenous rat brain vesicles

Dynamin is a GTPase thought to play a role in endocytosis based on genetic analysis of its homolog in Drosophila melanogaster shibire. Previous studies have stressed an in vitro association with microtubules, though additional evidence suggests that dynamin associates with membranous organelles. In an analysis of the enzymatic and membrane binding properties of dynamin, we have found that the acidic phospholipids, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol, are able to stimulate GTP hydrolysis in a manner similar to activation previously shown with microtubules. A neutral phospholipid, phosphatidylcholine, had no effect on dynamin GTPase. Activation of dynamin was biphasic, with a decrease in activity back to basal levels with increased concentrations of either microtubules or liposomes. A comparison between GTPase stimulation induced by microtubules and that by phospholipids suggests that ionic interactions between the basic C-terminal domain of dynamin and the negatively charged microtubule or phospholipid head group are important. In support of this, GTPase stimulation by these agents in combination was not additive. A salt-extracted membrane fraction from brain tissue also activated dynamin GTPase, though to a lower extent than pure phospholipids. These results suggest that membrane components could be responsible for some aspects of the regulation of dynamin function in vivo.

Regulation of the intracellular distribution of cytoplasmic dynein by serum factors and calcium

Previous work has indicated that cytoplasmic dynein localizes primarily to lysosomes in cultured fibroblasts, consistent with a function for dynein in retrograde movement. We now show that dynein can be redistributed from a lysosome-associated pool to a more diffuse cytoplasmic pool upon shifting fibroblasts to culture medium lacking serum for several hours. This effect on dynein localization is readily reversed upon addition of serum, with a substantial return to a control appearance of punctate staining within 10 minutes. The serum effect appears to be selective for dynein, in that the localization of kinesin and the overall morphology of intracellular organelles does not change. However, the distribution of kinesin-positive vesicles and lysosomes does appear to be altered during serum starvation, in that these organelles are located to greater extents in the peripheral regions of the cell. Dynein is also associated with the mitotic apparatus, but this localization does not change in response to serum starvation. Removal of calcium from the extracellular medium also results in the loss of punctate dynein staining, which can be recovered upon addition of calcium to calcium-free medium. The redistribution of dynein observed under these experimental conditions may reflect the activity of a regulatory process controlling the association of dynein with organelles, thereby providing one means of modulating intracellular transport.

Immunolocalization of cytoplasmic dynein to lysosomes in cultured cells

Polyclonal antisera have been raised against cytoplasmic dynein purified from calf brain and rat testis. These antibodies reacted most strongly with the 74 kDa dynein intermediate chain, but also recognized the 410 kDa heavy chain, and the 150 and 45 kDa polypeptides previously observed to copurify with cytoplasmic dynein from rat tissues. Localization studies were performed by indirect immunofluorescence microscopy using a fibroblast cell line. Dynein-specific staining appeared vesicular, distributed throughout the cell, but more concentrated near the nucleus. Double-labeling studies using fluorescent markers for membranous organelles indicated a co-localization of dynein with lysosomes. The distribution of the dynein-positive lysosomes was disrupted by treatment of the cells with microtubule-active drugs, and by acidification of the cytoplasm. Comparison of the distribution of lysosomes with peripheral microtubules indicated a high degree of coincidence. These results are consistent with the hypothesis that cytoplasmic dynein is involved in retrograde-directed movement of membranous organelles. In mitotic cells, dynein staining was also apparent along the microtubules of the mitotic apparatus, though vesicular staining was still conspicuous. The presence of dynein on vesicles as well as on spindle microtubules indicates that dynein distribution between these compartments may be regulated by distinct binding proteins.

Molecular cloning of the microtubule-associated mechanochemical enzyme dynamin reveals homology with a new family of GTP-binding proteins

A complementary DNA encoding the D100 polypeptide of rat brain dynamin--a force-producing, microtubule-activated nucleotide triphosphatase--has been cloned and sequenced. The predicted amino acid sequence includes a guanine nucleotide-binding domain that is homologous with those of a family of antiviral factors, inducible by interferon and known as Mx proteins, and with the product of the essential yeast vacuolar protein sorting gene VPS1. These relationships imply the existence of a new family of GTPases with physiological roles that may include microtubule-based motility and protein sorting.

Preparation of microtubules from rat liver and testis: cytoplasmic dynein is a major microtubule associated protein

A microtubule associated protein from brain tissue (MAP 1C), has been found to possess many properties in common with ciliary and flagellar dyneins (Paschal et al.:J. Cell Biol. 105:1273-1282, 1987). However, this protein, now designated as cytoplasmic dynein, exhibited several properties which distinguish it from axonemal forms of the enzyme. We have investigated these characteristics further in a study of cytoplasmic dyneins from non-neuronal tissues. Rat liver and testis in particular were found to contain high levels of cytoplasmic dynein. The yield of dynein from testis was over 70 micrograms/g of tissue, making this the best source of cytoplasmic dynein of all tissues so far examined. The characterization of dynein from these sources has confirmed and extended our previous observations concerning the unique properties of cytoplasmic dynein. Activation of liver and testis dynein occurred at low (less than 1 mg/ml) tubulin concentration. Polypeptides identified as subunits of brain cytoplasmic dynein (74, 59, 57, 55, and 53 kDa) were present in liver and testis preparations. In addition, polypeptides at 150 and 45 kDa were found to copurify with the non-neuronal dyneins. The liver and testis enzyme hydrolyzed pyrimidine nucleotides at rates up to 12.5 times faster than ATP, though the relative affinity of cytoplasmic dynein for CTP was much lower (Km = 1.0 mM) than that for ATP. The properties of the testis enzyme were consistent with its identification as a cytoplasmic dynein rather than a sperm axonemal precursor. These data indicate that cytoplasmic dyneins may be widespread in distribution and that they share certain biochemical properties unique from those of axonemal dyneins. These characteristics are consistent with the proposal that cytoplasmic dynein plays a universal role in retrograde organelle motility.

Isolated flagellar outer arm dynein translocates brain microtubules in vitro

The inner and outer arms of the flagellar axoneme generate the forces needed for flagellar movement; these arms contain ATPases called dyneins. To date, there has been no method for studying the mechanochemical transducing activity of isolated dyneins. Recently, it was found that the brain microtubule-associated protein (MAP) 1C is a microtubule-activated ATPase with the structural and force-producing properties of dynein. MAP 1C translocates microtubules in an in vitro gliding assay, suggesting that such an assay could also be used with axonemal dyneins. Here, we demonstrate that outer-arm dynein isolated from sea urchin (Strongylocentrotus purpuratus) sperm and adsorbed to a glass coverslip can translocate calf-brain microtubules along the surface of the coverslip. Our results conclusively demonstrate that outer-arm dynein by itself is capable of generating shearing forces. The ability to examine the force-generating properties of flagellar dynein in vitro should greatly facilitate studies of the mechanism of action of this important mechanochemical transducer.

Temperature-dependent reversible assembly of taxol-treated microtubules

Taxol is a plant alkaloid that binds to and strongly stabilizes microtubules. Taxol-treated microtubules resist depolymerization under a variety of conditions that readily disassemble untreated microtubules. We report here that taxol-treated microtubules can be induced to disassemble by a combination of depolymerizating conditions. Reversible cycles of disassembly and reassembly were carried out using taxol-containing microtubules from calf brain and sea urchin eggs by shifting temperature in the presence of millimolar levels of Ca2+. Microtubules depolymerized completely, yielding dimers and ring-shaped oligomers as revealed by negative stain electron microscopy and Bio-Gel A-15m chromatography, and reassembled into well-formed microtubule polymer structures. Microtubule-associated proteins (MAPs), including species previously identified only by taxol-based purification such as MAP 1B and kinesin, were found to copurify with tubulin through reversible assembly cycles. To determine whether taxol remained bound to tubulin subunits, we subjected depolymerized taxol-treated microtubule protein to Sephadex G-25 chromatography, and the fractions were assayed for taxol content by reverse-phase HPLC. Taxol was found to be dissociated from the depolymerized microtubules. Protein treated in this way was found to be competent to reassemble, but now required conditions comparable with those for protein that had never been exposed to taxol. Thus, the binding of taxol to tubulin can be reversed. This has implications for the mechanism of taxol action and for the purification of microtubules from a wide variety of sources for use in self-assembly experiments.

A microtubule-activated ATPase from sea urchin eggs, distinct from cytoplasmic dynein and kinesin

We report an ATPase activity, present in sea urchin egg cytosol, that is activated by microtubules. The activity sediments at 10 S in sucrose gradients and is clearly distinct from activities at 12 S and 20 S due to cytoplasmic dynein. Potent activation of the ATPase is observed when endogenous egg tubulin is induced to assemble with taxol or when exogenous taxol-stabilized pure brain tubulin microtubules or flagellar outer-doublet microtubules are added. No activation by tubulin subunits or taxol alone is detectable. In contrast to flagellar or cytoplasmic dynein, the microtubule-activated enzyme is unaffected by vanadate or by nonionic detergents and hydrolyzes GTP in addition to ATP. In contrast to kinesin, it cosediments with microtubules in the presence or absence of ATP. The microtubule-activated enzyme may have a role in microtubule-based motility.

Isolation of mitotic microtubule-associated proteins from sea urchin eggs

We have used a taxol-based microtubule purification procedure and monoclonal antibodies to isolate and characterize the MAPs of mitotic spindle microtubules in the fertilized sea urchin egg. In so doing, we hope to have identified some of the essential working parts of the mitotic apparatus, namely those proteins that regulate the assembly, disassembly, organization and mechanochemical properties of spindle microtubules. The results of this effort strongly suggest that a rich diversity of polypeptides associate with mitotic spindle microtubules. Whether each of these represents an individual protein species is not currently known. It is possible, for example, that particular spindle MAPs comprise multiple, distinct subunits. This would not be surprising in light of the facts that both MAP-1 and MAP-2 contain lower molecular weight subunits, and that axonemal dyneins are complex assemblies of several polypeptide species. Our future efforts with the sea urchin system will be to determine how the various mitotic spindle MAPs we have identified function individually and in concert, and how those functions contribute to the mechanochemical properties of the spindle.

Characterization of the sea-urchin egg microtubule-activated ATPase

We have found that cytoplasmic extracts from unfertilized sea-urchin eggs contain a prominent microtubule-activated ATPase activity. This activity is induced by polymeric tubulin, but not by tubulin subunits. The activity cosediments with taxol-stabilized microtubules in an ATP-independent manner. We have separated the ATPase from cytoplasmic dynein and other ATPases on sucrose gradients. The sedimentation, enzymic and microtubule-binding properties of the microtubule-activated species show it to be distinct from cytoplasmic dynein, myosin and kinesin. Since the major function of microtubules in the early sea-urchin embryo is in mitosis, this enzyme represents a new candidate for a role in spindle motility.

Use of multiple monoclonal antibodies to characterize the major microtubule-associated protein in sea urchin eggs

Microtubules assembled from sea urchin eggs with the use of taxol contain a 77,000-dalton protein as the major nontubulin component [Vallee and Bloom (1983): Proc Natl. Acad. Sci. U.S.A. 80:6259-6263]. We have raised five monoclonal antibodies to this protein to aid in its characterization. Immunoblot analysis of the sea urchin microtubule purification fractions indicated that the protein copurified quantitatively with microtubules. All five antibodies stained the mitotic spindle of dividing sea urchin eggs by immunofluorescence microscopy, indicating that the protein was a component of the mitotic spindle and suggesting that it was actually localized on microtubules in vivo. Immunofluorescent staining of higher resolution was observed in a subpopulation of the coelomic cells found in adult sea urchins, confirming that the 77,000-dalton protein is indeed present on microtubules in vivo. Because taxol was not used for the immunofluorescence experiments, we conclude that the microtubule-associated protein (MAP)-like behavior of the 77,000-dalton protein in vitro was not induced artifactually by taxol. To determine whether this protein is a component of sea urchin microtubules in general, cilia obtained from blastula stage embryos and sperm tail flagella were analyzed with the antibodies. The protein was undetectable by both immunoblot analysis and immunofluorescence microscopy in both preparations of axonemal microtubules. These results indicated that the 77,000-dalton MAP is restricted to cytoplasmic and mitotic microtubules in the sea urchin. Furthermore, in view of its particular abundance in embryos, whose microtubules are devoted substantially to mitosis, the 77,000-dalton MAP is likely to play an important role in regulating the activity of mitotic spindle microtubules in the sea urchin.

Determination of vitamin A and vitamin A acetate by high-performance liquid chromatography with fluorescence detection

A rapid and sensitive high-performance liquid chromatographic method with fluorescence detection has been devised for the analysis of vitamin A (retinol) and its acetate. The method employs a C18 reversed-phase column and methanol as an eluent. The detection of these two compounds is monitored with fluorescence excitation at 348 nm and emission at 470 nm. Detector noise established the lower limit of quantitation at approximately 0.5 ng. Plasma samples were employed to evaluate the accuracy, reproducibility, and applicability of the method. Less than 1 ng of vitamin A in plasma (as low as 1 microliter) can be quantitated by this procedure.

Rapid diglyceride phosphorylation in isolated bovine rod outer segments

When isolated bovine rod outer segment fragments were incubated with [gamma-32P]ATP, 32P, as revealed by autoradiography, was rapidly incorporated into rhodopsin bands on sodium dodecyl sulfate polyacrylamide gels, and into a low Mr lipid band. Incorporation of 32P into rhodopsin was light-dependent, but labeling of the lipid band was not. A single phosphorylated product, phosphatidic acid, was identified by 2-dimensional thin layer chromatography and by high pressure liquid chromatography of the corresponding glycerophosphate ester. Incorporation of label into phosphatidic acid was detected as early as 15 sec following start of incubation and the product was stable for at least 30 min. No other products were detected, indicating that under the experimental conditions phosphatidic acid was not metabolized to other phospholipids. Up to 1 mol phosphatidic acid was formed per 18 to 40 mol rhodopsin present.

Identification of phosphatidylinositol kinase in rat liver lysosomal membranes

Liver lysosomes from Triton-injected or normal rats were found to rapidly incorporate 32P from [gamma-32P]ATP into a lipid component of the membrane, in vitro. The lipid was identified as phosphatidylinositol 4-phosphate based on its chromatographic behavior on Silica Gel H thin layer plates as compared with standard phosphoinositides. The deacylation product, glyceryl-phosphorylinositol phosphate, was compared with standards in chromatographic and electrophoretic systems to further substantiate the identification of the radioactive material. A trace of phosphatidylinositol 4,5-bisphosphate was also found. The properties of the lysosomal membrane phosphatidylinositol kinase were examined using both endogenous lipid and exogenous phosphatidylinositol as substrate. The enzyme was active at neutral pH in the presence of 20 mM MgCl2. The addition of 0.4% Triton X-100 stimulated the enzyme activity toward endogenous substrate, and the highest activity was observed in the presence of detergent and 1 mM phosphatidylinositol. Degradation of the product was seen only in the presence of Triton X-100. The specific activity of the lysosomal phosphatidylinositol kinase is comparable to the detergent-stimulated activity of liver microsomes and plasma membrane, the previously recognized sources of this enzyme in the liver cell.

Characterization of endogenous protein phosphorylation in isolated rat liver lysosomes

Membranes prepared from highly purified rat liver lysosomes contain endogenous protein-phosphorylation activities. The transfer of phosphate to membrane fractions from [gamma-32P]ATP was analyzed by gel electrophoresis under acidic denaturing conditions. Two phosphopeptides were detected, with molecular weights of 3,000 and 14,000. Phosphorylation of these proteins was unaffected by the addition of cAMP, cGMP, or the heat-stable inhibitor of cAMP-dependent protein kinase. No additional phosphorylation was observed when cAMP-dependent protein kinase was included in the reaction or when exogenous protein kinase substrates were added. The 14,000-dalton 32P-labeled product was formed rapidly in the presence of low concentrations (250 microM) of either Ca2+ or Mg2+. This product was labile under both acidic and alkaline conditions, suggesting that this protein contains an acyl phosphate, present presumably as a catalytic intermediate in a phosphotransferase reaction. The lower molecular weight species required a high concentration (5 mM) of Mg2+ for phosphorylation, and micromolar concentrations of Ca2+ stimulated the Mg2+-dependent activity. The addition of Ca2+ and calmodulin stimulated the phosphorylation reaction to a greater extent than with Ca2+ alone. This activity was strongly inhibited by 0.2 mM LaCl3 and to a lesser extent by 50 microM chlorpromazine or trifluoperazine. These results suggest that the 3000-dalton peptide may be phosphorylated by a Ca2+, calmodulin-dependent kinase associated with the lysosomal membrane.

Fast charge movements in skeletal muscle fibres from Rana temporaria

1. Fast charge movements were measured in cut skeletal muscle fibres from Rana temporaria.2. The initial time course of the current in response to a sudden displacement of the membrane potential from -110 to -60 mV was analysed in terms of an electrical equivalent circuit modified from Falk & Fatt (1964).3. The specific resistance in series with the sarcolemma was estimated as 7.4 Omega cm(2). The total capacity (surface sarcolemma plus tubular membrane) was estimated as 3.43 muF/cm(2).4. The asymmetry currents settling within 1 ms during depolarizing pulses of increasing size (on-response), from a holding potential around -120 mV, could be described in terms of a single exponential. The asymmetry currents after the pulses (off-response) exhibited at least two components.5. The integral of the on-response, Q(on), as a function of V(p), could be fitted using a function of the Boltzmann type. At the mid-point of the distribution curve, equal to -38 mV, the slope was 0.012 mV(-1). A saturating value of 28 pC was reached at 40 mV.6. The off-response to pulses not exceeding 3 ms exhibited two components. The first one had an exponential time course. The charge Q(off) associated with this fast component was always equal to Q(on).7. tau(on) (the relaxation time constant), as a function of membrane potential was asymmetrical, exhibiting a maximum value of 233 mus at about -38 mV.8. For V(p) values smaller than -20 mV the Q(on)-V(p) and tau(on)-V(p) curves could be analysed using the two-state transition model. From this analysis the average transition potential V' was estimated as -38 mV and the effective valence of the mobile charges as 1.36.9. Double-pulse protocols (duration of pre-pulses referred to as T in the range 0-3 s) showed that Q(on) and tau(on) decreased as T increased. Single transient analysis shows that the changes are confined to the transient for depolarizing pulses.10. This immobilization of the charges is reversible and follows a similar time course to the slow inactivation of the Na(+) conductance described in the preceding paper.11. A differential effect of the depolarizing pre-pulse on the ionic and asymmetry currents is seen in the decrease of tau(on) with increasing T while tau(m) remains constant.

Temperature dependence of the sodium channel gating kinetics in the node of Ranvier

Temperature dependence of the Na+ channel gating kinetics was measured from the ionic and charge displacement currents in the node of Ranvier of Xenopus laevis, m3h kinetics was applied, assuming a delay, delta t, in the activation process. The rate constants for the m- and h-process showed Arrhenius temperature-dependence with Q10 of 2.34 and 2.9 respectively, while delta t exhibited non-Arrhenius temperature-dependence. Q10 for PNa, measured as 1.6, was smaller than for the rate constants and similar to that for a diffusion process. A negative shift and decrease in voltage sensitivity of the steady-state curves, h infinity and m infinity, occurred with decreasing temperature. The maximum time constant obtained from a single exponential fit to the displacement currents during the pulse for times greater than 90 microseconds exhibited Q10 of 2.01, which lies between that for PNa and that for tau m.

Activation and inactivation characteristics of the sodium permeability in muscle fibres from Rana temporaria

1. The steady-state and kinetic characteristics of the processes of activation and inactivation of the Na(+) permeability, P(Na), were measured in cut skeletal muscle fibres from Rana temporaria under voltage-clamp conditions.2. The specific resistance, r(ss), in series with the surface sarcolemma, was estimated as 6 Omega cm(2) by measuring the initial value of the membrane potential transient in response to current pulses under current-clamp conditions. To reduce the error in the potential across the sarcolemma introduced by r(ss), Na(+) currents were recorded using positive feed-back compensation, in the presence of tetrodotoxin (2.4-5 nm).3. P(Na)(t) was fitted with m(3)h kinetics assuming a voltage-dependent delay, deltat, to the start of the activation process.4. The P(Na)-V(p) curve exhibited saturation at potentials more positive than 30 mV. m(infinity), calculated as (P(Na), (infinity)/ P(Na))((1/3)) as a function of V(p), was a sigmoid curve with a mid point at -35 mV. The slope, dm(infinity)/dV(p), at this point was 0.032 mV(-1).5. Using a double-pulse protocol a non-exponential time course for the development of fast inactivation at small depolarizations was observed.6. The time constant for activation, tau(m), as a function of V(p), and tau(h) as a function of V(p), could be fitted with an approximately bell-shaped function, maximum of 430 mus at -43 mV and 925 mus at -78 mV respectively, at 15 degrees C.7. The mid-point potential of the h(infinity)-V(l) curve occurred at -58 mV, and h(infinity) approached 1 for V(1) values more negative than -103 mV.8. Using a double-pulse procedure the development of a slow inactivation of the Na(+) current was demonstrated. Its time course could be described in terms of a single exponential function, time constant equal to 0.58 s. The recovery from slow inactivation could be described by a similar exponential for recovery times smaller than 1 s.